Thermoacoustic loudspeaker



Oct. 22, 1968 J. s. ARNOLD THERMOACOUSTIC L OUDSPEAKER 2 Sheets-Sheet 2Filed Jan. 8, 1965 United States Patent ice I 3,407,273 1THERMOACOUSTIC, LQUDSPEAKER 1 James S. Arnold, Palo Alto, .'Calil.,assignor to Staiiford "Research Institute, MenlofPark,"'Calif., acorporation ofCalifornia i FiledJan'.8,1965, SenNo. 424,414

-- 3 Claims. (Cl. I79-108)' -ABSTRACT-OF THE DISCLOSURE 1 A modulatedflow acoustic device is provided 'wherein the flow of acombustible gasis'mo'dulated in acedrdance with signal'inform'ationl The modulated'gasstream is burned whereby sonic energy is produced at a higher soundintensity than is obtainable in the absence of combustion.

This invention relates to modulated'ga's fiowf loudspeakers, and moreparticularly "0 a rnethod 'and means for increasing the'acoiisticaloutput thereof. *rr modulated gas flow loudspeaker ijs'a device whichadV cepts a steady flow of gas and -modulates the gas fiow rate (increasesand decreases) ina manner cohtinuously res' onsive tofa' control signal.The gas flow: variations produce an acousticaloutput. p In order toincrease the acoustical output of a modulated flow acoustic'devic'eg'on'e increases'themagn'itude "of the gas flow rate or thecontrol signal. However, for any given device, electrical andmechanical; considerations limit the output which one can obtain byincreasing the g'as 'flow and control signal inputs. r An object" ofthis invention 'is to proyideian arrange: ment whereby acoustical outputof any givenrnodulated flow acoustic device can be increased'with'utincreasing the gas flow rate or control signal. I v r v g Yet anotherobject of the present invention isthe p'roa vision ofa nov'elandimproved constru'etion fora lib 7, speaker whereby it'sacousticialoutput isjncr'eased extem sively over that heretofore obtainable with agiven loudspeakerconstruction, A 1;: 1

A further object of the preseh'tinventionisthe; vision ofa novel andimproyed constfuction for a; wher by its acoustical output i s increasedextensive that heretofore obtainable'withagivensire f The foregoing andother objects of this invention may be achieved .in a construction for amodulatedfiow acoustic device whereby flow of a combustible gas ismodulated inv accordance with the signal information After modu theg'gasstream is burned. This adds energy fromth bustion to the sonic energy toproduce ahighehs ium intensity than would be obtained in the absence ofcoin-' bustion. j p I The novel features that areconsideredcharacteristic ofthis invention are set forthwithparticularityxin-the appended claims. The invention itself both as toits organization and method of-operation,-as'well as additional objectsand advantages thereof, will bestbe understood from the followingdescription when read in connection with the accompanying drawings, inwhich:

FIGURE 1 is a cross-sectional view of a loudspeaker structure which ismodified in accordance with this invention;

FIGURE 2 is a cross-sectional view of another embodiment of theinvention; and

FIGURE 3 is a view in cross-section of a modulated flow acoustic device,such as a siren, modified in accordance with this invention.

Reference is now made to FIGURE 1 which comprises a cross-sectional viewof a loudspeaker construction in accordance with this invention. Atypical loudspeaker 10, which may be of the electrodynamic type, ismounted in 3,407,273 Batented Oct. 22, 1968 the usual rear closure 12,in a manner well known in the art. The rear closure 12 is preferablymade of metal. An

7 opening is provided therein so that leads 14 may extend I the closure12. The closure member 20 is attached to the rear closure 12 as bybolts22'. I

The portion of the rear surface 24 which is opposite to the diaphragm 26of the loudspeaker isgiven the shape of a frustrum of a cone'so that itsubstantially parallels the diaphragm, and also is spaced therefrom. Thecenter of the cover member 20 has an opening 27therein which has theshape of two truncated cones intersecting at their smaller ends in asmall straight section 28, with the smaller cone being on the sideadjacent the base of the diaphragm 26 and the larger side opening out ofthe cover 20. A metal screen30 is placed 'in'the' opening 29 adjacentthe outside edge. A passageway 32 is provided in the closure member 20which couples the space: between the diaphragm and the back side of theclosure 20 to'the outside of the container. The source of combustiblegas 34 is connected by means of a pipe or hose 36 to the outside end ofthe opening 32. A valve 38 which is adjustable, controls the flow of gasfrom the source of combustible gas through the passage 32 into the spacebetween the rear surface 24 and the diaphragm 26.

In operation, the signal source 16 provides electrical signals whichdrive the loudspeaker 10. Gas from the source 34 is enabled to flow inthe space provided adjacent the diaphragm so that it may be modulated bythe vibrations of the diaphragm in response -to the signal source.

1 thermal augmentation of the acoustic output of the loudspeaker. Theamount of augmentation may be optimized by adjustment of the flow rateof the combustible gas using the valve 38. In: an embodiment of theinvention which was built and successfully operated, a mixture of airand natural gas was employed as the combustible gas.

It was found that not only the flow rate adjustment, but also the air togas ratio of the mixture affected the extent of the augmentation of theacoustic output. These adjustments may easily be monitored by using amicrophone appropriately located in front of the loudspeaker, the

output 'from the microphone being applied to a voltmeter or sound levelmeter. In the embodiment of the invention which was built, when theoptimum adjustment was obtained, the measured sound pressure level wasmore than 10 db higher than with no combustion but otherwise identicalconditions.

FIGURE 2 illustrates in cross-section another embodi ment of thisinvention whereby a loudspeaker operating along the same principles asthe one shown in FIGURE 1 is loaded by an acoustic horn 42. In thisembodiment of the invention, there is provided the conventional magnet44 having a circular slot therein within which the voice coil 46 isplaced and supported by a voice coil support 48. The voice coil 46 isactually wound on a bobbin 50 which is attached to the voice coilsupport 48. The voice coil is driven by signals received from a signalsource 52.

The magnet 44, voice coil and support are placed within a container 54having a top wall 54A, the inner side form a passageway 56 therewith. Anopening is placed in the center of the top wall 54A. A gas flowsmoothing member 58 is placed on the top of the magnet '44concentrically with the voice coil 46. 'The gas flow smoothing memberhas side walls which are substantially parallel and almost coextensivewith the bobbin. Then the walls curve inwardly in-a generally conicalmanner to a point. The-walls of the opening in the top wall 54A-curve ina somewhat parallel manner with the walls of the gas flow smoothingmember to define an exit passageway 60 therebetween. The areaaof thispassageway, 'taken perpendicular to the gas flow, varies with distancealong it a.

in substantially thesame manner -as' the law'of expansion ofthehorn 42.

The horn 42 is placed on the top wall 54A with the opening in its throatmatching that of the exit passageway 60. An inlet opening 62 in the sidewall of the container 54 admits a combustible gas into the passageway56. The signal source provides signals to the voice coil which togetherwith the bobbin 50 respond thereto by moving more or less into and outof the passageway 56. Thereby, the gas flow through the passageway ismodulated by'the signals from the signal source 52. The flame holdingscreen 64 may be advantageously located within the small cross-sectionportion of the horn by controlling gas fiow rate and mixture ratio. As ageneral principle, at this location the flame velocity willapproximately equal the rate of gas outflow.

The horn serves the function of increasing the efficiency anddirectivity of radiation.

FIGURE 3 shows in cross-section another embodiment of this inventionwherein a conventional siren 70 is employed. Gas, instead of air, is fedfrom a source of combustible gas 72 into an opening in the case 74 ofthe siren 70. The gas flow is modulated by the rotating chopper wheel 76which is within the case. The rotating chopper wheel is driven from anexternal drive source, represented here by a drive shaft 78. Themodulated gas is urged by the rotating chopper wheel toward an outletopening 80 in the case, over which is placed the acoustic horn 82containing the flame supporting screen 86. The acoustic output of themodified siren is increased considerably when the gas is ignited andburned. A

Other arrangements of this invention may be provided wherein fuel andoxidant gases are kept separate and the flow of only one component ismodulated. Mixture can take place after modulation, but before ignition.This may be done in any of the structures shown in FIGURES 1, -2 or 3 byfeeding one of the gases into. the region between the throat of theacoustic horn and below the flame supporting screen by means of a pipeconnection to a source ofsupply. i

. Because combustible gases are used, the thermoacoustic loudspeakermust employ safety devices to stop gas flow ifthe flame goes out.Flashback prevention and ignition means must also be provided. Suchdevices are well known in the combustion art.

'There has accordingly been shown herein a novel and useful method andmeans of increasing the output of an acoustic device by using it tomodulate a combustible gas which is then ignited.

1. The improvement in a'loudspeaker of the type having a diaphragm whichis actuated responsive to signals to modulate the air surrounding saiddiaphragm comprising aclosureirnember spacedfrom saiddiaphragm todefingabhamber therebetw'een, a source of. combustible gas,', .an ppenin'gat, oneffedge of said; closure .rnember extending between said chamberand its outside-edge, a source of combustible gas, meansfor-lintroducingcombustible gas through'said opening into said chamber, a second openingin said closuregnember extending outward from the region of said closuremember in said chamber over'the "center of'sziid diaphragm, a 'fiameholding. screemdisposed insaid second opening near the outside edge ,ofsaid closure member, whereby when said combustible,gaapassing throughsaid flame holding screen ia ignited the acoustic output of saidloudspeakeris increased. A

.,A .thermoacoustic loudspeaker comprising a diaphragm, means to actuatesaid diaphragm responsive, to signals for modulating the. airsurrounding said diaphragm inaccordance with said'signals, means formingan enclosure with said diaphragm, said enclosure forming means havingwalls spacedopposite from and substantially parallel to said diaphragm,a source'of combustible gas rneans for introducing combustible gas fromsaid source intosaid enclosure, a, gas escape openingrin said i encloureforming means positioned over the center of said diaphragm, whereby thevibration of said diaphragm determinesthe arnount of gas passing out ofsaid gas escape means, and means for supporting a flame at the outsideend of said gas escape opening whereby when said combustible gas isignited the acoustic noutput of said loudspeaker is increased.

3. In combination a loudspeaker having a diaphragm and means forvibrating said diaphragm responsive to electrical: signals, a rearclosure means for said loud speaker comprising a boxhavinganopening inone wall thereof the size of said, diaphragm, means mountingsaidloudspeaker in said jboxwith said diaphragm facing outwardly within saidopenings,' a front closure means mounted on said boxover saidloudspeaker diaphragm,

said front closure means having walls opposite said dia-' phragm. whicharespaced therefrom and substantially parallel thereto and .dcfiningtherewith a modulating en: closure a source of combustible gas, meansfor introducing 'said's source'of combustible gas at a location of saiddiaphragm displaced from the center thereof into said modulating'chamber, and afgas escape 'DPQDF IS through said enclosure memberextendingfrom the region thereof over,the center of said diaphragmoutwardly, whereby 'when said combustible gas escaping from said escapeopening is ignited the output of said loudspeaker s i d. p i vReferences Cited i UNITED STATESPATENTS 969,037 8 1934 R ieber 116-137KATHLEEN H. CLAFFY, Primary Examiner.

A. MC QGILL', Assistant Examiner.

